1. Field of the Invention
This invention relates to electrical switching apparatus and, more particularly, to circuit interrupters, such as, for example, aircraft circuit breakers providing are fault protection. The invention also relates to methods for detecting arc faults.
2. Background Information
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
Subminiature circuit breakers are used, for example, in aircraft electrical systems where they not only provide overcurrent protection but also serve as switches for turning equipment on and off. Such circuit breakers must be small to accommodate the high-density layout of circuit breaker panels, which make circuit breakers for numerous circuits accessible to a user. Aircraft electrical systems usually consist of hundreds of circuit breakers, each of which is used for a circuit protection function as well as a circuit disconnection function through a push-pull handle.
Typically, subminiature circuit breakers have only provided protection against persistent overcurrents implemented by a latch triggered by a bimetal responsive to I2R heating resulting from the overcurrent. There is a growing interest in providing additional protection, and most importantly arc fault protection.
During sporadic arcing fault conditions, the overload capability of the circuit breaker will not function since the root-mean-squared (RMS) value of the fault current is too small to activate the automatic trip circuit. The addition of electronic arc fault sensing to a circuit breaker can add one of the elements required for sputtering arc fault protection—ideally, the output of an electronic arc fault sensing circuit directly trips and, thus, opens the circuit breaker. See, for example, U.S. Pat. Nos. 6,710,688; 6,542,056; 6,522,509; 6,522,228; 5,691,869; and 5,224,006.
It is believed that known arc fault detection methods could not be used with the smallest class of aerospace breakers because of space constraints. Consequently, a new electronics approach with minimal component count is needed, which meets space requirements and, also, offers relatively lower cost and higher reliability.
Known digital arc fault detection techniques for aerospace systems operate periodically at regular intervals. That is, at the end of each electrical half-cycle, the detection circuit performs the same sequence of events, collecting information about the currents and voltages in the electrical system and processing this information using various algorithms. If the rules specified by the algorithms indicate that a hazardous condition exists, then the arc fault detection circuit requests the circuit breaker to interrupt the current.
In these arc fault detection circuits, the half-cycle periodicity results from use of a voltage zero-crossing detector, which generates a logic level square wave synchronized to the line voltage. Whenever the square wave changes state (at voltage zero crossings), an interrupt occurs within a microprocessor. The microprocessor then collects information about the state of the electrical system, performs one or more algorithms, and issues a trip request, if needed.
However, aerospace circuit breakers are relatively small compared with most residential, commercial and industrial circuit breakers. The printed circuit board area available for arc fault detection electronics in the smallest aerospace circuit breakers is minimal. As such, it is believed that known arc fault detection techniques cannot be implemented in the smallest aerospace circuit breaker package style because there is simply not enough printed circuit board space to accommodate the required electronics.
There is room for improvement in arc fault circuit interrupters and in methods for detecting arc faults.